1. Field
The disclosure relates to a method, system, and article of manufacture for storage management in the cascaded replication of data.
2. Background
Information technology systems, including storage systems, may need protection from site disasters or outages, and features for data migration, data backup, and data duplication. Implementations for disaster or outage recovery, data migration, data backup, and data duplication may include replication of data in storage systems. Such mirroring or replication of data may involve interactions among hosts, storage systems and connecting networking components of the information technology system.
Synchronous replication is an operation that allows the shadowing of application system data from a first site to a second site. In synchronous replication, the updates performed by a host application to the volumes at the first site are synchronously shadowed onto the remote volumes at the second site. In synchronous replication, write updates are ensured on both copies (the copy at the first and second sites) before the write is considered to be completed for the host application. Furthermore, in synchronous replication the host application does not get the “write complete” condition until the update is synchronously done in both the first and the second site. Therefore, from the perspective of the host application the data at the volumes at the second site is equivalent to the data at the volumes at the first site.
Synchronous replication increases the response time as compared to an asynchronous replication, and this is inherent to the synchronous operation. The overhead comes from the additional steps that are executed before the write operation is signaled as completed to the host application. Also synchronous replication between the first site and the second site may be comprised of signals that travel through the links that connect the two sites, and the overhead on the response time of the host application write operations will increase proportionally with the distance between the two sites. Therefore, the distance affects a host application's write response time. In certain implementations, there may be a maximum supported distance for synchronous replication operations referred to as the synchronous communication distance.
In an asynchronous replication, updates of the volumes of a first site onto the volumes of a second site are performed in an asynchronous manner, while the host application is running. In asynchronous replication, the host application receives a write complete response before the update is copied from the volumes of the first site to the volumes of the second site. In this way, a host application's write operations are free of the typical synchronous overheads. Therefore, asynchronous replication is suitable for remote copy solutions at very long distances with minimal impact on host applications. There is no overhead penalty upon the host application's write such as in synchronous replication. However, asynchronous copy operations may not continuously maintain an equivalent copy of the data of the first site at the second site.
Many customers are interested in a 3-site replication solution in which the first and second sites, referred to as local and intermediate sites respectively, are within “metro distances” of each other, where in certain situations “metro distances” are distances that are less than 300 km. In other situations, “metro distances” could be less than a value that is different from 300 km., e.g., “metro distances” could be defined as distances that are less than 250 km. The third site, also referred to as a remote site, is at a “continental distance” from the intermediate site, where “continental distances” are distances that are 300 km. or more in certain situations. In other situations, “continental distances” could be more than or equal to a value that is different from 300 km., e.g., “continental distances” could be defined as distances that are 250 km. or more. However in all situations, continental distances are greater than metro distances. Synchronous replication is used between two storage devices at the local and intermediate sites, where the local and intermediate sites may be designated as site-A and site-B respectively. Asynchronous replication is used between the storage devices in the intermediate and remote sites, where the intermediate and remote sites are designated as site-B and site-C respectively. In such a 3-site replication solution, if any one site is not operational, there is no data loss, and a user can choose to recover data and/or operations at either of the two remaining sites. In such solutions, a volume at site-A may be referred to as an A-volume, a volume at site-B may be referred to as a B-volume, and a volume at sited C may be referred to as a C-volume.